The present invention relates to an exhaust emission purification system for a diesel engine which cleans up particulate matter (hereinafter, referred to as PM) and NOx (nitrogen oxide) in exhaust gas from an engine such as a diesel engine.
As regards discharge of PM to be discharged from the diesel engine, a restriction has been enhanced together with NOx, CO (carbon monoxide), unburned hydrocarbon and the like year by year, and there has been developed a technique by which this PM is collected by a filter called xe2x80x9cDiesel Particulate Filtersxe2x80x9d (hereinafter, referred to as DPF) to reduce an amount of PM to be discharged outwardly.
For DPF for directly collecting this PM, there are a wall flow type monolithic honeycomb filter made of ceramic, a fiber type filter obtained by making ceramic or metal into a fibrous shape, and the like. An exhaust emission purification device using these DPFs is arranged in an exhaust pipe of an engine to clean up exhaust gas to be generated by the engine.
In this DPF, however, clogging develops as PM is collected and exhaust gas pressure (exhaust pressure) is increased, and therefore, it is necessary to remove the PM collected by this DPF, and several methods and systems have been developed.
Among them, there are a system for burning and removing PM by heating the filter with an electric heater or a burner, and a system for inverse-washing by flowing air in the opposite direction. In the case of these systems, however, since energy for heating is supplied from the outside to burn PM, there are a problem for worsening fuel consumption and a problem that it is difficult to control regeneration.
Also, when any of these systems is adopted, in the majority of cases, the structure is arranged such that there are provided two lines of exhaust passages each having a filter, and collection of PM and regeneration of the filter are alternately repeated in each exhaust passage. For this reason, the system becomes large and the cost is also prone to be increased.
In order to cope with these problems, there has been proposed a continuously regenerating type DPF system in which regeneration temperature of the filter is lowered to reduce an amount of energy supplied from the outside, and exhaust heat from the engine is utilized to regenerate the filter. In this system, a wall flow type filter and a catalyst are combined. This filter is constructed such that a multiplicity of exhaust passages (cells) whose periphery is enclosed with a porous wall surface are formed and an inlet side and an outlet side of these exhaust passages are sealed in a zigzag shape, respectively.
In this system, since regeneration of the filter and PM collection are basically performed continuously and an exhaust passage becomes one line of system, control of regeneration is also simplified. This system has the following three types.
A first type is a nitrogen dioxide regenerating type DPF system, and is composed of an oxidation catalyst on the upstream side and a wall flow type filter on the downstream side. Through the use of this oxidation catalyst such as platinum, NO (nitrogen monoxide) in the exhaust gas is oxidized. Through the use of NO2 (nitrogen dioxide) generated by this oxidization, PM collected by the filter is oxidized into CO2 (carbon dioxide) to remove the PM. The oxidation of PM due to this NO2 has lower energy barrier than the oxidation of PM due to O2 (oxygen) and is performed at, low temperature. For this reason, thermal energy in the exhaust gas is utilized, whereby PM can be oxidized and removed while the PM is continuously being collected to regenerate the filter.
Also, a second type is an integrated model nitrogen dioxide regenerating DPF system, and is an improvement of the first system. This system is formed by coating a wall surface of the wall flow type filter with the oxidation catalyst. This wall surface performs both oxidation of NO in the exhaust gas and oxidation of PM due to NO2. Thereby, the system is simplified.
Thus, a third DPF system with a PM oxidation catalyst is formed by a precious metal oxidation catalyst such as platinum and a wall flow type filter with PM oxidation catalyst obtained by coating the wall surface with PM oxidation catalyst This wall surface oxidizes PM at lower temperature. This PM oxidation catalyst is a catalyst for directly oxidizing PM by activating O2 in the exhaust gas, and is formed of cerium dioxide or the like.
In this third system, in a low-temperature oxidation region (350xc2x0 C. to about 450xc2x0 C.), PM is oxidized with NO2 through the use of a reaction in which NO of oxidation catalyst is oxidized into NO2. In a medium-temperature oxidation region (400xc2x0 C. to about 600xc2x0 C.), through the use of PM oxidation catalyst, O2 in the exhaust gas is activated to oxidize PM through the use of a reaction in which PM is directly oxidized. In a high-temperature oxidation region (about 600xc2x0 C. or higher) higher than temperature at which PM burns with O2 in the exhaust gas, PM is oxidized with O2 in the exhaust gas.
In these continuously regenerating type DPF systems, through the use of the oxidation reaction of PM due to the catalyst and NO2, the temperature at which PM can be oxidized is lowered.
On the other hand, in exhaust gas from the diesel engine, the exhaust gas temperature varies as shown in FIG. 8 depending upon a load and a number of revolutions of the engine. For the reason, the DPF is not always in an optimum temperature state. When the exhaust gas temperature is in the low-temperature region, the activity of the catalyst decreases, and the PM cannot be sufficiently oxidized. Accordingly, there is a problem that it is difficult to have an excellent PM cleanup performance over the entire engine operating region.
As one of measures against this problem, there is an exhaust emission purification device for a diesel engine proposed in Japanese Patent Application No. 155894/2001 by the present inventor.
As shown in FIG. 5, this exhaust emission purification device is provided with: a first continuously regenerating type DPF 12A in an exhaust passage 9 of the engine; a bypass passage 101 on the upstream side of this first DPF 12A; a second continuously regenerating type DPF 13A provided in this bypass passage 101; and exhaust gas temperature rise means. This exhaust gas temperature rise means uses an intake throttle valve (intake shutter) 22, an exhaust gas introduction mechanism and an exhaust throttle valve (exhaust shutter) 23. In an engine operating state during idling or the like in which exhaust gas temperature is low, the exhaust gas temperature is raised to 300xc2x0 C. or higher, and the exhaust gas is caused to flow into a bypass passage 101 to treat the exhaust gas through the use of a second continuously regenerating type DPF 13A.
In this exhaust emission purification device, a second continuously regenerating type DPF 13A consisting of the oxidation catalyst 131 and DPF 132 shown in FIG. 6 is, as shown in FIG. 5, arranged at a position closest to an exhaust manifold 4. This arrangement causes exhaust gas whose temperature has been raised by the exhaust gas temperature rise means to pass through the second continuously regenerating type DPF 13A before its temperature is not lowered even in the low exhaust gas temperature operating state.
With control of a change-over valve 102, when the exhaust gas temperature is within a predetermined low-temperature region, the exhaust gas is caused to pass through a second continuously regenerating type DPF 13A in the vicinity of the exhaust manifold, and when the exhaust gas temperature is within a predetermined high-temperature region, the exhaust gas is caused to flow through the first continuously regenerating type DPF 12A. Thereby, in the entire operating state region of the engine, the temperature of the exhaust gas passing through each DPF 122, 132 is set to 300xc2x0 C. or higher.
The temperature of exhaust gas passing through each of these DPFs 12A and 13A is maintained at high temperature, whereby the activity of the oxidation catalyst 121, 131 is maintained and a reaction for oxidizing NO to NO2 or the like is secured. Thereby, in the broad operating region of the engine, PM collected by DPF 122, 132 is reliably continuously oxidized to clean up the PM efficiently and reliably.
In this respect, since this second continuously regenerating type DPF 13A is employed when the exhaust gas temperature is low, and as an operating state of this engine, the operating state is at comparatively low load in many instances, the exhaust flow rate itself is less and the amount of PM in the exhaust gas is also comparatively small. For this reason, the capacity of the second continuously regenerating type DPF 13A can be set to smaller capacity than the first continuously regenerating type DPF 12A to be disposed in the exhaust passage.
This exhaust emission purification device, however, has a problem that it is not possible to sufficiently clean up NOx to be contained in the exhaust gas and NOx to be generated by an oxidation reaction in the continuously regenerating type DPF because a NOx catalyst 14 for cleaning up NOx is provided only behind the first continuously regenerating type DPF 12A.
In other words, for main NOx catalyst for use currently, there are SCR catalyst (selective contact catalyst) and NOx absorber reduction catalyst, and either has a region indicating a high rate of cleanup being 280xc2x0 C. or higher as their characteristics are shown in FIGS. 3 and 7. For the reason, when any of these catalysts is employed, since in the engine operating state during idling and an operation at low load, the temperature of exhaust gas for flowing through this NOx catalyst becomes 100xc2x0 C. to 200xc2x0 C., the catalyst activity decreases, a rate of cleanup of NOx becomes 0 to 25%, and NOx cannot sufficiently be decreased.
It is an object of the present invention to provide an exhaust emission purification system for a diesel engine capable of efficiently cleaning up not only PM but also NOx even in an engine operating state at low exhaust gas temperature such as an idling operation and a low load region operation and exhibiting excellent exhaust gas cleanup performance in an engine broad operating region.
In order to achieve the above-described object, there is provided an exhaust emission purification device for a diesel engine according to the present invention, comprising:
a first exhaust emission purification device, arranged in an exhaust passage of an engine, having a first continuously regenerating type diesel particulate filter and a first SCR catalyst;
a bypass exhaust passage placed side by side in an exhaust passage on the upstream side of the first exhaust emission purification device;
a second exhaust emission purification device, arranged in the bypass exhaust passage, having a second continuously regenerating type diesel particulate filter and a second SCR catalyst;
reducing agent supply means for NOx cleanup arranged upstream of a diverging portion of the bypass exhaust passage;
exhaust gas temperature region detection means for detecting an engine exhaust gas temperature region;
exhaust gas temperature rise means for raising exhaust temperature by means of combination of an intake throttle and an exhaust introduction mechanism for opening an exhaust passage of a cylinder during an intake stroke;
channel switching means for controlling the flow of exhaust gas to the bypass exhaust passage; and
control means for controlling the exhaust gas temperature rise means and the channel switching means correspondingly to an exhaust gas temperature region obtained by detecting by the exhaust gas temperature region detection means, wherein
the control means is constructed such that when the exhaust gas temperature region detection means detects that the exhaust gas temperature is within a predetermined low-temperature region, the exhaust gas temperature rise means raises the exhaust gas temperature, and the channel switching means switches the channel in such a manner that the exhaust gas flows through the bypass exhaust passage, and after causing the exhaust gas to pass through the second exhaust emission purification device, causes the exhaust gas to pass through the first exhaust emission purification device.
For these first and second continuously regenerating type DPFs, there can be employed a nitrogen dioxide regenerating type DPF consisting of an oxidation catalyst on the upstream side and a wall flow type filter on the downstream side, an integrated type nitrogen dioxide regenerating DPF system to be constituted by a wall flow type filter with catalyst obtained by coating the wall surface with an oxidation catalyst, a DPF system with PM oxidation catalyst composed of a precious metal oxidation catalyst such as platinum and a wall flow type filter with PM oxidation catalyst obtained by coating the wall surface with a PM oxidation catalyst, or the like.
As a NOx catalyst for decreasing NOx, the SCR catalyst is employed. A rate of NOx cleanup of this SCR catalyst is very high as shown in FIG. 3.
Also, the channel switching means is constituted by an open-close valve provided in an exhaust gas passage which goes side by side with the bypass exhaust passage. Or, the channel switching means can be constituted by a channel switching valve provided in a diverged part from the exhaust passage of the bypass exhaust passage or in a junction part to the exhaust passage.
For the exhaust gas temperature region detection means, there can be employed means for detecting the exhaust gas temperature region on the basis of such map data as exemplified in FIG. 4 which has been set in advance from the load and the number of revolutions of the engine, means for directly measuring temperature of the exhaust gas through the use of a temperature sensor provided in the exhaust passage or the like.
According to an exhaust emission purification system for a diesel engine having this structure, the following effects can be exhibited.
When the exhaust gas temperature is beyond the scope of the low-temperature region, high-temperature exhaust gas passes through the first exhaust emission purification device to activate an oxidation catalyst, a PM oxidation catalyst and the first SCR catalyst of the first continuously regenerating type DPF. As a result, the PM and NOx are cleaned up at a high rate of cleanup by the first exhaust emission purification device.
Also, when the exhaust gas temperature is within the scope of the low-temperature region, the exhaust gas temperature rise means raises the exhaust gas temperature and the channel switching means flows the exhaust gas into the bypass exhaust passage. The exhaust gas thus raised therefore passes through the second exhaust emission purification device. This second exhaust emission purification device is arranged on the upstream side, and it is more difficult to cool the exhaust gas than the first exhaust emission purification device. Therefore, the exhaust gas at high temperature raised activates the oxidation catalyst, PM oxidation catalyst and the second SCR catalyst of the second continuously regenerating type DPF. As a result, the PM and NOx are cleaned up at a high rate of cleanup by the second exhaust emission purification device.
In other words, when the exhaust gas temperature is within the high-temperature region, the PM and NOx can be efficiently cleaned up through the use of the first continuously regenerating type DPF and the first SCR catalyst, provided in the exhaust passage. Also, in an engine operating state in which the exhaust gas temperature during idling, during a low-load operation or the like is low and the amount of exhaust gas is also small, the PM and NOx can be efficiently cleaned up through the use of the second continuously regenerating type DPF with small capacity and the second SCR catalyst with small capacity, provided in the bypass exhaust passage in the vicinity of the exhaust manifold.
In the exhaust emission purification system for the diesel engine, the predetermined low-temperature region is characterized by the exhaust gas temperature being 300xc2x0 C. or lower.
Also, in the exhaust emission purification system for the diesel engine, the second exhaust emission purification device is provided in the vicinity of the exhaust manifold or within the exhaust manifold. This structure enables the exhaust gas before the temperature lowers to flow into the second exhaust emission purification device.
Also, in the exhaust emission purification system for the diesel engine, the predetermined low-temperature region is divided into a first low-temperature region and a second low-temperature region lower than the first low-temperature region, and the structure is arranged such that when the exhaust gas temperature region detection means detects that the exhaust gas temperature is within the first low-temperature region, the exhaust gas temperature rise means controls to throttle back an intake throttle valve, and that when the exhaust gas temperature region detection means detects that the exhaust gas temperature is within the second low-temperature region, the exhaust gas temperature rise means controls to throttle back the intake throttle valve, and controls to introduce the exhaust gas into the cylinder during an intake stroke.
This structure enables exhaust gas at exhaust gas temperature suitable for activation of each catalyst with more caution and attentiveness to details to be supplied.
Also, in the exhaust emission purification system for the diesel engine, the structure is arranged such that when it controls to throttle back the intake throttle valve, the exhaust gas temperature rise means also control to throttle back an exhaust throttle valve. This structure will enable the exhaust gas temperature to be further raised.
In the general engine operating condition, therefore, the rate of cleanup for not only PM but also NOx can be greatly improved. Particularly, since the exhaust passage is switched depending upon the engine operating condition to thereby keep the temperature of the SCR catalyst through which the exhaust gas passes at 300xc2x0 C. or higher at all times, it is possible to remarkably improve the rate of NOx cleanup, and theoretically, the NOx will be able to be 100% removed.